This invention relates to a lens assembly and an image sensing device ideal for use in a video camera system of the type having an interchangeable lens assembly.
Automatic focusing (AF) used in video equipment such as video camera relies upon the so-called “hill-climbing method” according to which focusing is performed by extracting high-frequency components from a video signal obtained from an image sensing device such as a CCD and driving a camera lens so as to maximize a mountain-shaped curve representing the characteristic of the high-frequency components.
This automatic focusing method is advantageous in that special optical members for focusing are not required and focusing can be performed accurately regardless of whether distance to the subject is long or short.
An example in which such an automatic focusing method is used in a video camera whose lens assembly can be interchanged will be described with reference to FIG. 9.
A conventional variable-power lens assembly 916 includes a variable-power lens 902 and a compensating lens 903 mechanically connected by a cam. When zooming is performed manually or under power, the variable-power lens 902 and compensating lens 903 move in unison. The variable-power lens 902 and compensating lens 903 are collectively referred to as zoom lenses. In a lens system of this type, a front lens 901, which is situated closest to the subject, serves as the focusing lens and is moved along the optic axis to perform focusing.
Light that has passed through this group of lenses forms an image on the image sensing surface of an image sensing device 904 in a camera body 917, whereby the incident light is photoelectrically converted to an electric signal and outputted as a video signal.
The video signal is applied to a CDS/AGC circuit 905, which is composed of a correlated dual-sampling circuit and an automatic gain control circuit. The CDS/AGC circuit 905 samples and holds the video signal and then amplifies the signal to a predetermined level. The amplified signal is converted to digital video data by an A/D converter 906 and these data are then applied to a processing circuit, which is the next stage of the camera, to be converted to a standard television signal.
The video signal that has been converted to the digital video data by the A/D converter 906 enters a bandpass filter (hereinafter referred to as a “BPF”) 907.
The BPF 907 extracts high-frequency components, the level of which changes in dependence upon the state of focusing, from the video signal, and a gate circuit 908 picks out only a signal that corresponds to a portion that has been set on a focal-point detection area on a screen picture. A peak-hold circuit 909 holds the peak of the signal at intervals synchronized to a whole-number multiple of a vertical synchronizing signal and generates a focus (AF) evaluation value.
This AF evaluation value is accepted by an AF microcomputer 910 in the camera body 917. The AF microcomputer 910 decides a focusing motor speed conforming to the degree of focusing and decides focusing motor drive direction in such a manner that the AF evaluation value will increase. The speed and direction of the focusing motor are sent to a lens microcomputer 911.
The lens microcomputer 911 performs focusing by driving the focusing lens 901 along the optic axis, this being achieved by driving a motor 913 via a motor driver 912 in response to a command from the microcomputer 910.
The microcomputer 910 decides the driving direction and speed of the zoom lenses 902, 903 in conformity with the status of a zoom switch 918 and sends these signals to a zoom motor driver 914 in the lens assembly 916 to drive the zoom lenses 902, 903 via a zoom motor 915.
The camera body 917 is detachable from the lens assembly 916 so that a different lens assembly may be connected.
The image sensing apparatus shown in FIG. 9 is capable of having its lenses interchanged and for this reason the controls for automatic focusing are provided in the camera 917. Consequently, when the response of automatic focusing is decided so as to be optimum for a specific lens, there are occasions where the respones will not be optimum for another lens. It is difficult to realize optimum performance with regard to all lenses capable of being attached to the camera.
The applicant has previously proposed an image sensing apparatus in which the controls for automatic focusing are provided on the side of the lens assembly and a focusing signal necessary for the purpose of executing focusing control is delivered from the body of the image sensing apparatus to the lens assembly.
Automatic focusing described above relies upon a mechanism in which the image sensing apparatus such as a camera automatically judges the photographic conditions and adjusts lens position to achieve a state construed to be suited to the photographic conditions. As a result, situations can arise in which the intentions of the photographer are not reflected in the video obtained.
For example, consider a situation in which a subject in the distance and a subject close by are both present in the area of the screen picture. If automatic focusing is performed on the basis of information representing the entirety of the screen picture in which the images are currently appearing, one of the plurality of subjects mentioned above will be brought into focus. However, the image sensing apparatus cannot determine whether this is the main subject that the photographer wishes to focus upon. In order to avoid such situations as much as possible, the general practice is to use a technique in which emphasis is placed upon measuring the distance to the subject located at the center of the screen picture (this is referred to as “weighted-center distance measurement”) and automatic focusing is executed based upon the results of measurement.
The reason for this is that when the photograph performs photography, often the main subject is located in the center of the picture. However, if the main subject is located somewhere other than the center of the picture, there are instances where focusing cannot be carried out properly with respect to the main subject. This is the chief disadvantage of this system.
To improve upon this, the specification of Japanese Patent Application No. 4-1541656 discloses an image sensing apparatus in which the photographer looking at the finder is capable of selecting the main subject by his or her line of sight in such a manner that the main subject will be brought to the best focus regardless of where it is located in the screen picture. In accordance with this line-of-sight position detection distance measurement method, it is possible for the position of the main subject to be changed at will while limiting the distance measurement area.
The positioning designating means for selecting the main subject is not limited to line-of-sight detecting means. For example, it is possible to conceive of position designating means which decides direction of movement and position by synthesizing amount of movement in two dimensions using a pointing device such as a joystick or mouse.
In general, the distance measurement area in the weighted-center distance measurement” method is set to be large relative to the screen in such a manner that a subject not located in the center will be focused appropriately. With the line-of-sight position detection distance measurement method, the distance measurement area is set to be small relative to the screen so that competing subjects near and far will not coexist in the distance measurement area. This is so that the camera can be directed toward the main subject to achieve optimum focus regardless of where the main subject is located in the screen picture.
However, when it is attempted to realize the function for selecting the main subject with the interchangeable-lens image sensing apparatus having the automatic focusing controls provided in the lens assembly, the distance measurement area on the screen changes in conformity with a change in the position of the main subject and so does the focusing signal extracted from the distance measurement area. Accordingly, whether a change in the focusing signal is ascribable to a change in the subject distance that accompanies a camera operation such as panning or to a change in the distance measurement area that accompanies a change in the position of the main subject cannot be distinguished merely by delivering the focusing signal from the body of the image sensing apparatus to the focusing control section of the lens assembly.
For example, when a person walking from left to right on the screen is followed by position designating means described above, the focusing signal changes with movement of the subject even though there is no change in the distance to the subject. As a consequence, the lens microcomputer performs a focusing operation upon erroneously judging that the subject has gone out of focus. This causes inadvertent blurring of the subject.
Further, in the example described above, the lens assembly is incapable of recognizing the distance measurement method. Consequently, the automatic focusing operation is unstable in the line-of-sight position detection distance measurement method, the foundation of which is a small distance measurement area that is readily susceptible to the influence of camera operation and changes in the subject, the result of which is frequent changes in the focusing signal. Thus, if an attempt is made to control automatic focusing on the side of the lens assembly in the camera system described above, many problems arise in terms of control between the camera and lens assembly.